1. Field of the Invention
The present invention relates to a method of etching a gate stack in the formation of semiconductor devices and, more particularly, to a method and system for etching multiple layers in a gate stack in order to prepare a structure of 25 nm in size or less.
2. Description of Related Art
In material processing methodologies, pattern etching includes the application of a patterned mask of radiation-sensitive material, such as photoresist, to a thin film on an upper surface of a substrate, and transferring the mask pattern to the underlying thin film by etching. The patterning of the radiation-sensitive material generally involves coating an upper surface of the substrate with a thin film of radiation-sensitive material and then exposing the thin film of radiation-sensitive material to a radiation source through a reticle (and associated optics) using, for example, a photolithography system. Then, a developing process is performed, during which the removal of the irradiated regions of the radiation-sensitive material occurs (as in the case of positive photoresist), or the removal of non-irradiated regions occurs (as in the case of negative resist) using a base developing solution, or solvent. The remaining radiation-sensitive material exposes the underlying substrate surface in a pattern that is ready to be etched into the surface. Photolithographic systems for performing the above-described material processing methodologies have become a mainstay of semiconductor device patterning for the last three decades, and are expected to continue in that role down to 65 nm resolution, and less.
The resolution (ro) of a photolithographic system determines the minimum size of devices that can be made using the system. Having a given lithographic constant k1, the resolution is given by the equationro=k1λ/NA,  (1)where λ is the operational wavelength, and NA is the numerical aperture given by the equationNA=n·sin θo.  (2)Angle θo is the angular semi-aperture of the system, and n is the index of refraction of the material filling the space between the system and the substrate to be patterned.
To print smaller and smaller structures, current lithographic trends involve increasing the numerical aperture (NA). However, although the increased NA permits greater resolution, the depth of focus for the images projected into the light-sensitive material is reduced, leading to thinner mask layers. As the light-sensitive layer thickness decreases, the patterned light-sensitive layer becomes less effective as a mask for pattern etching, i.e., most of the (light-sensitive) mask layer is consumed during etching. Without a dramatic improvement in etch selectivity, single layer masks have become deficient in providing the necessary lithographic and etch characteristics suitable for high resolution lithography.
An additional shortcoming of single layer masks is the control of critical dimension (CD). Substrate reflections at ultraviolet (UV) and deep ultraviolet (DUV) wavelengths are known to cause standing waves in the light-sensitive layer due to thin film interference. This interference manifests as periodic variations in light intensity in the light-sensitive layer during exposure, resulting in vertically spaced striations in the light-sensitive layer and loss of CD.
In order to counter the effects of standing waves in the light-sensitive layer as well as provide a thicker mask for subsequent pattern etch transfer, a bilayer or multilayer mask can be formed that incorporates a bottom anti-reflective coating (BARC). The BARC layer includes a thin absorbing film to reduce thin film interference; however, the BARC layer can still suffer from several limitations including poor thickness uniformity due in part to spin-on deposition techniques.
A hard mask may also be used to provide improved maintenance of critical dimensions. The hard mask may be a vapor deposited thin film provided under the light sensitive layer to provide better etch selectivity than the light sensitive layer alone. This etch selectivity of the hard mask material permits use of a thinner mask that allows greater resolution while also allowing a deeper etch process. The present inventors have recognized, however, that the use of conventional hard masks have limited etch selectivity and resilience to etch processes that will limit their use in future generation devices with even smaller structures.